A common form of metal detectors transmits a transmit magnetic field and receives a receive magnetic field radiated by elements of the environment that are influenced by that transmit magnetic field. Examples of this form of metal detectors include detectors that are used for detecting explosive landmines, unexploded ordnance, precious native metals, coins, jewellery and caches of similar items, in ground. Development of metal detectors has involved not only the improvement of sensitivity to sought objects (targets) but also include, through processing signals due to the receive magnetic field radiated by the sought objects, the separation of signals due to their radiated fields from signals due to fields radiated by objects in which the detectorist has less interest, as well as the classification of the nature of the sought objects.
Transmission, reception of the receive magnetic field and processing of the signals due to the receive magnetic field are effected through electronic circuitry.
These electronic metal detectors usually include transmit electronics for generating a repeating transmit signal cycle of a fundamental period applied to an inductor, also known as a transmit coil, that transmits a changing magnetic field often referred to as a transmit magnetic field.
Metal detectors contain receive electronics which processes a receive signal from an induced voltage due to a receive magnetic field to produce an indicator signal, the indicator signal at least indicating the presence of at least some sought objects (for example, metallic targets such as gold, jewellery etc.) within the influence of the transmit magnetic field.
There is a problem with such an operation, whenever the receive signal includes a signal due to the presence of one or more desired metallic objects, it is often mixed with, or contaminated by, unwanted signals, for example signals due to unwanted buried objects (such as ferrous objects), background noises, and signals due to soil or ground. Indeed, it is possible that a receive signal that is composed entirely of unwanted signals will be able to elicit the indication of a target, even when there is no desired target present.
In metal detectors that classify detected targets as well as merely detecting them, contaminated receive signals, if not further processed, can result in erroneous classification of the targets, for example, by classifying a non-ferrous object as a ferrous object. This can further lead to reduced sensitivity to small or deeply-buried sought objects. Acceptable detection performance can be difficult to achieve without further processing of the receive signal.